"First Light" for 3.6 m Telescope

"First Light" for 3.6 m Telescope
A first look at the sky through the ESO 3.6 m telescope with
its mirror aluminized was taken during the night of November 7-8. The "first light" to a telescope is a unique moment,
and it was celebrated with a midnight tea. But apart from
that, we had many troubles that night. Obviously you cannot expect everything to work at once in an instrument as
complex as the 3.6 m telescope; much time must be
allowed for debugging the entire system. The people who
work with it have to learn how to handle the various procedures, but after a very successful aluminization, we had
nearly forgotten that something could go wrong. And so,
during the night of first light, the images (what a disaster!)
looked like sm all hearts. Very romantic, although not
exactly what astronomers are looking for in the sky. It took
us two days to find and cure that fault. The 11-ton main
mirror had chosen to rest on two ofthe back supports only.
From the third one it kept a respectful distance of 1 mm.
The third night (November 9-10) was much more successful. The newly-discovered dwarf galaxy in Sculptor
(see page 16) was on the programme, and a one-hour exposure revealed its beauty and showed a good number of
its brighter stars resolved. This first plate has been followed by others of up to two hours exposure (see the
cover photo). During the first five nights we have taken
about 30 plates and the image quality looks very good. We
still have to determine the limiting magnitude, but it
should be at least 24 m, possibly even fainter.
A lot of work has been done to getthis far. In the last issue
of the Messenger we reported that the mechanical assembly of the telescope was completed. Since then, the telescope controls have been put to work by a team from the
Electronics Group from Geneva who spent more than a
month on La Silla.The optics have been installed and a systematic alignment and test programme has been carried
out by a team of three from the Optics Group in Geneva d uring the last two months. As mentioned above, the mirror
has been aluminized. Not a small job either, but it was so
weil prepared that already the first aluminizing gave an excellent result. On these big jobs as weil as on hundreds of
sm aller ones, the work has been progressing steadily, although not without minor setbacks. On August 26, a light-
The ESO 3.6 metre telescape on La Silla. In the front end the prime-focus cage.
2
A look down in the prime-focus cage. To the right, the astronomer's chair, from which he guides the telescope during the expoSures. In the centre the adaptor with an eyepiece for direct viewing
and focussing. Above this the TV-camera (see text).
Preparing observations. The astronomer is about to enter the
prime-focus cage, which he rides during the exposure. To facilitate entry, the telescope is brought to horizontal position. The
exchange of top-ends, wh ich is a unique feature of the ESO telescope, is also done in this position.
ning struck the dome and caused a lot of damage to the
electrical installation and the newly-installed electron ics. A
few weeks later apart of the building was flooded with oil,
and cleaning-up took several days. Never mind, we feel
that the photos of the last nights more than compensate
the difficulties behind!
Now, however, we should be careful not to give the im-
pression that our work on the telescope is finished. So far,
only the prime focus is in operation. A great many improvements and minor jobs still have to be made. But in
between, the observations contin ue. And it is our belief and
hope that the percentage of time devoted to astronomy will
from now on steadily increase.
S. Laustsen, November 12, 1976
Optical Alignment of 3.6 m Telescope and First Tests
The Optics Group from Geneva has been intensively occupied for the last ten weeks with the alignment and testing of
the prime-focus optics for the 3.6 m telescope.
The basic alignment of the optics of the telescope perpendicular to the declination axis was completed about
three weeks ago. Since that time, an intensive period of
Hartmann testing has fully occupied uso
The measuring facilities at present available on La Silla
are not sufficiently accurate to give a final figure for the
concentration of geometrical energy in a given diameter.
However, there ist clear evidence that the specification of
75 % within a diameter of 0.4 arcsecond should be fulfilled-we think probably by a clear margin. The computer
analysis of the plates shows that the basic, lower-orderaberration terms are smalI; while the workshop tests had
already establ ished that the su rfaces are very smooth. Turbulence effects in the dome and telescope seem, at present, to be the factors limiting quality and the precision of
centring. However, even with the existing plate-measuring
facilities, it has been possible to centre the system to within
0.2 arcsecond of tangential coma, in spite of dome turbulence and indifferent extern al seeing.
External seeing has been mainly poor d uring the whole
test phase, but the first photographs with the telescope
have shown very circular images of faint stars on IIla-J
plates with diameters of 1 to 11/ 2 arcsecond. With the
actual seeing conditions prevailing, the Hartmann tests are
at least an order of magnitude more precise than visual or
photographic assessments.
As soon as the Hartmann plates have been measured on
a more accurate measuring machine in Europe, a complete
report of the test results will be published. These results
will refer to the naked mirror and to the complete primefocus system with the Gascoigne plate correctors. The triplet correctors will be available in a few months and will be
the subject of a further report.
The Cassegrain-focus alignment and tests should take
place about next March.
R. Wilson, October 29, 1976
The Prime-Focus Cage
The first plates have now been taken with the 3.6 m telescope. This was done in the prime-focus cage that allows
the astronomer to ride in the front end of the telescope
during the observations. In the following we shall explain
how the cage was equipped for the first test of the telescope.
In the cage there is room for one astronomer, an adaptor
and some auxiliary equipment needed by the astronomer
3